Field of the Invention
The present invention is concerned with novel films that can be used as a dielectric layer in a capacitor.
Description of the Prior Art
When power is needed for time periods of minutes or more, batteries can deliver the needed stored power, or the power can be used as generated. However, when time frames are in the second to sub-second time frame, storage capacitors become important. Advanced technology applications such as directed energy weapons platforms typically need power bursts from a few milliseconds to sub-microseconds. All but a select few capacitor designs have difficulty supplying power in those time frames. These capacitors are known as pulsed power capacitors. Pulsed power capacitors operate at very high voltages, 10-100 kV, but with very little intrinsic energy, often in the nanofarad range or less. However, in the time frame needed they can deliver 1 megaAmp or more, depending on configuration. This is far more current than can be spontaneously generated in that time frame.
Pulsed power capacitors for directed energy weapons have specific needs. Because the intent is to use them for weapons platforms, the power systems need to be mobile. Additionally, delivering the power needed requires a lot of space. Currently, the capacitor bank needed to power a single rail gun is enormous. In addition to the shear volume of the capacitors, there is also a primary power generation unit, charging systems, switching units, pulse inductors, high current wiring, and cooling systems needed for the operation of the rail gun power system in addition to all the needed systems for the physical rail, targeting systems, round loading, and fire control. Other directed energy systems, such as the Air Force's airborne maser, laser, and EMP systems, have significantly greater limitations to volume and mass of the power supply and operational systems to maintain an airborne system. These tactical needs actually apply to all cases, weapons or other types, where the system is intended to be mobile. In cases where the power system is stationary, much of the need moves away from power/energy density and to reliability and response. Regardless of the application, working to reduce the volume and mass of the total system is critically important.
At high energy levels, a spontaneous uncontrolled discharge of even a single capacitor unit can have catastrophic consequences. A graceful failure mechanism is required. Therefore, capacitors of this type employ what is known as “self-clearing” attributes. When there is a failure, typically a voltage breakdown, a corona release, or an electrical short between the plates, the energy released is sufficient to heat the “defect” site to vaporization. This means the complete vaporization of the metal plates, the dielectric film, and the defect generator takes place. The result is a small clear hole with a corresponding reduction in the area of the plates, but a capacitor that continues to work with a slight reduction of energy capacity. A capacitor is considered damaged and needs to be replaced when the stored energy is reduced by 5% of the original capacity.
The current state of the art of films for polymer dielectric separators in high power capacitors suffers from low dielectric constants, energy loss, low breakdown voltages, thermal instability, high-char yields from internal discharge events, and/or combinations of these issues. Improvement in the energy and power density of the capacitors can be accomplished by the increase in the dielectric constant of the film. A larger dielectric constant film allows for more electrons, and thus more energy to be stored onto the plates with the same voltage across the plates. Organic polymers typically have dielectric constants between 2 and 10. Some extreme examples can be found as high as 50. Inorganic types of insulators can have dielectric constants as high as 50,000. However, there is a trade-off to increasing the dielectric constant. First, as the dielectric constant goes up, the breakdown voltage typically decreases. The more polarizable the electrons in the material, the more likely conduction will occur from those electrons. Typically, the more polarizable the materials, the longer it takes for the induced dipole in the dielectric to relax and give the full power of the capacitor. For pulsed power applications, if relaxation is too slow, insufficient power is generated or the polarization of the capacitor can temporarily reverse. Lastly, the higher the dielectric constant the more the dielectric constant changes with respect to temperature. This is especially true for polymeric materials that generate much of their dielectric constant from their morphology.
The breakdown voltage of an insulating material is the minimum voltage that causes the material to become conductive. In a capacitor, the higher the voltage that can be applied across a film, the higher the resulting power output. A higher voltage that can be applied per unit thickness also generates more power per unit space. Thus, a higher breakdown voltage leads to higher power density. Currently the highest known breakdown voltage material is biaxially-oriented polypropylene (BOPP), with a breakdown voltage of 900 V/μm. This high breakdown voltage has placed BOPP as the market leader for mobile, pulsed power capacitor dielectrics and as a major player in other markets. However, BOPP only has a dielectric constant of 2. So, it can store a large amount of voltage for power delivery, but not as much energy.
BOPP has another property that has made it one of the materials of choice for pulsed power capacitors. It doesn't leave any char upon pyrolysis. When there is a defect and an internal discharge event, the polymer completely vaporizes and doesn't leave any conductive carbon material to further short the plates. This behavior is critical to the self-clearing effect. Many of the beneficial properties of BOPP are from the oriented morphology of the polymer. The film casting process aligns and stretches the polymer chains in the plane of the film. However, if the material reaches its Tg of about 105° C., the polymer chains relax, orientation is irreversibly lost, and the film shrinks, rendering the capacitor useless, and resulting in catastrophic failure if the capacitor is energized.
Polymer dielectrics that maintain performance criteria over a wider range of temperatures are desirable, because they require less thermal control systems. While this may not reduce the size of the specific capacitor used in an application, it may reduce the size of the ancillary cooling systems. Such reduction in overall systems will also help reduce the overall power system size, weight, and energy consumption. Many polymers can handle high temperatures, however, most of these polymers contain a large content of aromatic ring groups as part of the chemistry. This high degree of aromaticity often is what imparts the thermal stability to the polymer. Unfortunately, this aromaticity also impacts the self-clearing behavior of the polymer. When a polymer with a large content of aromatic groups is pyrolized through an internal discharge event, often there is residual carbonacious material. This material is a sufficient conductor of electricity to short circuit the capacitor plates, rendering the whole of the capacitor irreparably damaged. Improving dielectric material properties such as the dielectric constant, breakdown voltage, and thermal stability in materials through densification, utilization of materials not reliant on morphology, and/or locking in the morphology with crosslinking can improve the material aspect of the dielectric.